Industrial Automation networks provide specialized control systems for industry equipment, and thus enable the automation of various industrial processes. For example, such industrial processes may include plant measurement control or precise motion control in a robotized factory assembly line. As illustrated in FIG. 1, Industrial Automation networks 10 may comprise a plurality of slave devices 12, for example actuators or sensors, which are connected to a control device 14. In this example, each of the slave devices 12 is configured to transmit data to and receive data from the control device 14.
Various applications for Industrial Automation exist, which have different requirements in terms of latency and time synchronization between the network devices 12, 14. For the most time critical applications, isochronous real time communication between the slave devices 12 and the control device 14 is typically required. This demands very precise synchronization between the network clocks (not shown) at the respective network devices 12, 14, for example synchronization within one to a few microseconds.
There are a number of network protocols for Industrial Automation networks, one of which is the PROFINET IO protocol defined in IEC61784-2. PROFINET IO is based on Ethernet, and has the advantage that it offers a flexible communication model, which can carry both real-time and non-real time traffic. For the most time critical applications, which require isochronous real-time communication, PROFINET IO has defined the RT_Class_3. This class defines a communication cycle and, for each network device 12, 14, specifies a precise time slot in the communication cycle in which that network device 12, 14 may transmit data.
In order to synchronize the network clocks at the network devices 12, 14, to the necessary degree, PROFINET IO includes a Precision Time Control Protocol (PTCP). This protocol operates in the same manner as the Precision Time Protocol (PTP) standardized in IEEE 1588, based on the exchange of two-way time synchronization messages between pairs of network devices. However, the time synchronization messages in PTCP are encoded differently from those in PTP. FIG. 2 shows the mapping of PTCP and PTP messages.
By way of example, FIG. 3 is a signal diagram showing the transmission of PTP synchronization messages between a pair of network nodes (which may be referred to as network devices). In this example, a first network node (not shown) on the left of the page has a network clock acting as a “master” clock representing “master time”. A second network node 16 on the right of the page is synchronizing its network clock (which will be referred to as a “slave” clock) with respect to the master clock at the first network node.
At 300, the first network node transmits a first synchronization message to the second network node. The first network node includes in the first synchronization message, or in a follow up message, a first time stamp, t1, generated by its “master” clock, indicating the time of transmission of the first synchronization message. The second network node, at 310, receives the first time synchronization message and stores, together with the first time stamp, t1, a second time stamp, t2, generated by its “slave” clock indicating the time of receipt of the first time synchronization message (from the perspective of its slave clock). At 320, the second network node transmits a second time synchronization message to the first network node. The second network node stores a third time stamp, t3, generated by its slave clock indicating the time of transmission of the second time synchronization message. At 330, the first network node receives the second time synchronization message, and transmits a fourth time stamp, t4, generated by its master clock to the second network node, indicating the time of receipt of the second time synchronization message at the first network node.
Thus, the second network node 16 has four time stamps: two, t1 and t4 generated by the first network node by its “master clock”, and two, t2, t3, generated by the second network node by its “slave clock”. Thus, by assuming that the time of transmission of the first synchronization message (in the direction from the first network node to the second network node) is the same as the time of transmission of the second time synchronization message (in the direction from the second network node to the first network node), the second network node can calculate a time offset of its slave clock with respect to the master clock using the four time stamps: t1, t2, t3, t4. Thus, the second network node can adjust its network clock by the calculated time offset, so as to synchronize its network clock to the network clock of the first network node.
At present, Industrial Automation Networks are implemented using wired networks. For example, a typical topology of a PROFINET IO network is shown in FIG. 4. The network 10 comprises a plurality of IO (Input Output) devices 12, 14, coupled by wired connections 18 for example Ethernet cables.
However, the Applicant has appreciated that it may be advantageous to upgrade or implement Industrial Automation networks 10 such that the network devices 12, 14 can communicate over a wireless communications network.
Two possible wireless deployments are illustrated in FIG. 5. As shown in FIG. 5a, the IO (Input Output) devices 12, 14 could be configured to communicate via an operator controlled wireless communications network 20. Alternatively, as shown in FIG. 5b a dedicated wireless communications network 20 may be provided at the Industrial Automation site. In this case, a radio access network including a base station 22 and one or more core network nodes 24 may be located at the Industrial Automation site.
A cellular layout has the advantage that it may provide a flexible deployment, require less material handling, work-in-process inventory, and offer a reduced setup time in comparison to a wired network.
However, the use of a wireless communications network is not currently feasible, at least for the most time critical Industrial Automation applications. The 5G radio interface and related core network functions currently under development may be able to provide low enough latency to satisfy time critical Industrial Automation applications. However, the Applicant has appreciated that, in addition to low latency, the most time critical Industrial Automation applications require precise time synchronization between network devices, for example within one microsecond.
If PTP or PTCP synchronization messages are exchanged by network devices over a wireless communications network it is not currently possible to achieve sufficiently precise synchronization between the clocks at the network devices. This is because the frame structure of the radio interface, processing and buffering of transmissions by the User Equipments associated with the network devices, and scheduling at the base station, causes uncontrolled and varying asymmetries between the times of transmission of the synchronization messages. Thus, it cannot be assumed, as required by the PTP and PTCP time synchronization protocols, that the times of transmission of the time synchronization messages, in opposite directions, are the same.